1. Field of the Invention
The present invention relates to a press machine, and, more specifically, to a press machine such as a press brake used for bending sheet-like workpieces.
2. Description of the Prior Art
As is well-known, a bending press or press machine such as a press brake for bending sheet-like workpieces such as sheet metals is provided with a pair of upper and lower bending tools, one of which is so arranged as to be vertically moved by a ram toward and away from the other to bend a workpiece. Usually, the lower bending tool is fixed and the upper tool is mounted on the ram to be vertically moved toward and away from the lower bending tool. However, in some bending presses, the upper bending tool is fixed and the lower bending tool is movably held by the ram. In either case, the workpiece to be bent is placed or held on the lower bending tool so that it may be bent when either of the upper and lower bending tools, whichever is movable, is vertically moved toward the fixed tool. Also, the ram carrying the movable bending tool, in many bending presses, is hydraulically operated by a hydraulic means to vertically move the movable bending tool toward and away from the fixed bending tool.
In such a bending press, the bending angle to which the workpiece is to be bent is determined by the limit of travel of the ram to which position the ram is vertically moved to move the movable bending tool toward the fixed bending tool to thus bend the workpiece. Accordingly, it is necessary to accurately set the limit of travel of the ram or the movable bending tool so as to accurately bend the workpiece to a desired angle. In order to accurately control the position to which the ram is raised or lowered, in the case of a hydraulic press, a control valve is installed in the hydraulic circuit to raise or lower the ram. In addition, the press machine or bending press has a position determining member which controls the position to which the ram is raised or lowered by controlling the above-mentioned control valve; and the ram has an operation member which causes the valve to open when it is contacted by the position determining member. That is, when an operation member on the ram contacts the position determining member, the vertical movement of the ram is stopped in order to control its position.
It is desirable for the position determination to be done near the center of the ram. If it is done at one end, the equilibrium of the ram can be upset resulting in a large position error.
The above-mentioned position determining member may consist of a rotating link located on the frame side and at the center of the ram along its length so that it is free to rotate up and down. The operation member on the ram is designed to contact the tip of the rotating link. The position of the ram is determined by setting the angle of the above-mentioned rotating link. Consequently, the operation member, which moves up and down, contacts the rotating link the tip of which describes a circular arc. Since the relationship between the position at which the rotating link angle is set and the position at which the ram is set to stop is not proportional, it is very difficult to accurately set the position at which the ram stops in the vertical direction.
The position of the above-mentioned position determining member can be set either manually or automatically. Manual setting, which is done by rotating a handle provided on the side of the press machine, is difficult to operate accurately. Automatic setting, which is done by the ON/OFF operation of a pushbutton switch on a control box, is also hard to operate accurately.
Control of the speed of vertically moving the ram in a press machine or bending press is accomplished by opening and closing an unloading valve in the hydraulic circuit that drives the ram in the vertical direction. The above-mentioned unloading valve is opened and closed by stepping on a link-type foot pedal on the front of the press machine. This makes it impossible to operate the press machine from a remote location, which is a problem from the point of view of improving the safety of operation of the machine.
Another type of press machine or bending press uses a hydraulic fluid-filled cylinder under the foot pedal, and the pressure on the foot pedal is transmitted through the fluid to operate the unloading valve. However, this method also has problems: the hydraulic hose connected to the foot pedal is large in diameter and rigid, which makes operation difficult; and, when the machine is moved or transported, the foot pedal must be removed from the press machine allowing some of the hydraulic fluid to leak out and get on the floor and other nearby objects.
Still another problem with existing press machines or bending presses is that there is no detection device to directly detect the vertical position of the ram.